This invention relates to an improved process for preparing acrylic acid from propylene using a single reactor. In particular, the invention relates to a single reactor process for preparing acrylic acid from propylene utilizing an increased concentration of propylene reactant thereby providing increased capacity and throughput.
The preparation of acrylic acid from propylene generally proceeds in a vapor phase two step catalytic oxidation reaction. In the first step propylene is oxidized in the presence of oxygen, diluent inert gasses, water vapor, and appropriate catalysts to produce acrolein according to equation (I):
C3H6+O2C2H3CHO+H2O+heatxe2x80x83xe2x80x83(I). 
The acrolein is then oxidized, in a second step, in the presence of oxygen, diluent inert gasses, water vapor, and appropriate catalysts to form acrylic acid according to equation (II):
C2H3CHO+xc2xd O2C2H3COOH+heatxe2x80x83xe2x80x83(II). 
The two stage vapor phase catalytic oxidation of propylene to acrylic acid is generally performed using either tandem reactors wherein a separate reactor is utilized for each step (e.g., see the description in U.S. Pat. No. 4,873,368) or by utilizing one reactor to perform both steps (e.g., see the description in U.S. Pat. No. 4,526,783).
The acrylic acid prepared using such a vapor phase catalytic oxidation reaction is present in a mixed product gas exiting the reactor. Generally, the mixed product gas is cooled and is contacted with an aqueous stream in an absorption tower, thereby providing an aqueous acrylic acid solution from which acrylic acid can be isolated and purified. The remainder of the product gasses, known as the absorber waste gas or absorber off-gas, is incinerated or undergoes waste treatment. Depending on the reactants feed gas composition, the absorber off-gas may contain inert gasses, O2, water vapor, CO, CO2, unreacted propylene, unreacted acrolein and/or acrylic acid.
It is known in the art to recycle at least a portion of the absorber off-gas back to the reactor(s) to provide inert diluent gas and steam to the reactant composition. The propylene in the reactant composition must be diluted because at high propylene concentrations the reaction may proceed too quickly and become difficult to control. Recycle of the absorber off-gas provides the necessary diluent gasses and steam to the reactor feed to assure a suitable propylene concentration. In addition, recycling the absorber off-gas serves to reduce waste water generated by the process by reducing the amount of steam that is fed to the process. Furthermore, small amounts of unreacted propylene and acrolein contained in the off-gas are given another chance to react and thereby improve the overall acrylic acid yield by optimizing conversions of propylene and acrolein.
When absorber off-gas recycle is not used, steam and nitrogen are used as the primary diluents. Steam is not consumed, but may alter the selectivity, conversion and/or catalytic activity in the oxidation reactions and is part of the mixed product gasses emerging from the reactor. When the mixed product gasses are introduced into the absorption column, the steam substantially condenses tat the bottom of the absorption column and is a small part of the gasses flowing through the absorber.
However, a problem arises with absorber off-gas recycle. In contrast to the situation wherein absorber off-gas recycle is not used, a load develops at the top of the absorber because of the increased volume of inert gas flowing through the absorber. When absorber off-gas recycle is utilized, the off-gas is predominantly an inert gas such as nitrogen. When mixed product gasses containing such inert gasses are introduced into the absorber they do not generally condense at the absorber bottom, but rather remain part of the product gasses flowing through the absorber. Consequently, the increased inert gas content in the mixed product gasses introduced into the absorber causes an increase in the velocity of the gas flowing through the absorber. This results in a load at the top of the absorber. As the gas velocity gets higher, an increasing amount of product acrylic acid will remain with the absorber off-gas and be either lost to waste or be recycled back to the reactor. When it is recycled back to the reactor it can cause a decrease in catalyst activity. Consequently, regardless of whether it is lost to waste or recycles back to the reactor, the net result is a drop in acrylic acid yield.
A further problem results from the need to dilute the propylene in the gas feed to a manageable concentration. The dilution may be effected by absorber off-gas recycle or by adding steam and other inert materials or both. Because the two step oxidation of propylene to acrylic acid is highly exothermic, as the propylene concentration gets higher the danger of a runaway combustion increases. Also, the reaction mixture could become flammable and explode if ignited. Consequently, the oxidation of propylene to acrylic acid is generally practiced in the art utilizing a propylene concentration in the reactant gas feed composition of between 4 and 7 volume percent of the total reactant feed composition (see for example col. 2, lines 42-46 of U.S. Pat. No. 4,873,368). Accordingly, to assure control of the oxidation, propylene is diluted with steam and/or inert gasses such as nitrogen and combined with oxygen to form the feed composition. As a result, there is an additional load on the compressor which limits the capacity of the system. Consequently, any increase in capacity would require a larger compressor to handle the larger load.
As a result of the extra load on the absorber and on the compressor there is a limit on the capacity of the system which heretofore could not be remedied except by installation of larger equipment.
A further problem exists when tandem reactors are utilized. In tandem reactors there exists a high volume interstage between the two reactors through which the acrolein produced in the first reactor passes to the second reactor. This results in a longer residence time, compared to a single reactor, of the acrolein product in the interstage which may lead to homogenous reactions of acrolein and/or formation of foulants. Foulants may be formed by, for example, corrosion and deposition processes. Such homogeneous reactions are generally not catalytic, but rather are free radical reactions of acrolein which produce carbon oxides such as carbon dioxide and carbon monoxide, as well as other products such as acetaldehyde. Consequently, because of the longer interstage residence time in a tandem reactor process, steps such as cooling, reaction quenching, and acrolein dilution must be taken to reduce such homogeneous reactions of acrolein. In addition, the equipment and piping of the interstage is susceptible to gas leaks.
U.S. Pat. Nos. 4,365,087 and 4,873,368 have dealt with the problem of increasing process productivity/capacity by raising the propylene concentration level. However, the processes in these references used a tandem reactor process whereby either the temperature of the feed was limited ( less than 260xc2x0 C.), the oxygen to propylene ratio (1.1-2.0:1, preferably lower than 1.8) was kept low, additional oxygen and inert gas was fed to the second stage reactor, and the reaction was quenched somewhat before introduction to the second stage (""087) or the oxygen to propylene ratio (1.17-1.66:1) was even lower, additional oxygen and inert gas was fed to the second stage reactor, and the reaction was quenched somewhat before introduction to the second stage. Accordingly, the basis of the technique relied on two mechanisms for controlling the reaction at higher propylene concentrations:
(1) tightly controlling the temperature before entry into the first stage reactor and/or the second stage reactor; and
(2) limiting the amount of oxygen initially available to the first reactor for oxidation of propylene to acrolein and then adding more oxygen and diluent at the interstage before the second stage reactor so that the second reactor feed has a stoichiometrically sufficient amount of oxygen to allow suitable oxidation of acrolein to acrylic acid.
However, this technique is unavailable for a single reactor system because it is implausible to add further oxygen and inert gas and quench the reaction at the interstage because both reactions of equations (I) and (II) occur in each of the reactor tubes of the single reactor. U.S. Pat. Nos. 4,256,783 and 4,203,906 describe a single reactor system which is useful in a variety of catalytic vapor phase oxidation reactions including the preparation of acrolein and/or acrylic acid. However, the example relating to acrylic acid (see columns 9 and 10, Example 5 of the ""783 patent) does not utilize a reactant feed having a higher propylene concentration.
The present inventors have now discovered that with the single reactor system described herein it is possible to provide feeds to the reactor which contain a higher concentration of propylene than previously thought. Such higher concentration feeds are accomplished without the need to utilize a lower oxygen:propylene feed ratio, quenching of the reaction between stages and the consequent addition of oxygen and inert gas to the second stage to assure proper stoichiometry. Consequently, less absorber off-gas is required for dilution so that loads on the absorber and compressor are lightened resulting in an increase in capacity without additional capital expenditure.
Furthermore, a process is provided wherein homogeneous reactions of acrolein, as well as other interstage reactions, and interstage gas leaks are substantially eliminated.
Accordingly, a novel process for preparing acrylic acid from propylene is described herein wherein the following advantages are provided:
(1) increased throughput/capacity is provided without additional capital expenditure;
(2) downstream debottlenecking is realized through producing an aqueous acrylic acid stream in the absorber having a higher concentration of acrylic acid because less water is condensed and less acrylic acid is lost overhead in the absorber;
(3) since there is less water condensed in the aqueous acrylic acid there is a reduction in the waste generated by the process;
(4) less system energy is required because of the reduced compressor load;
(5) there is a lower pressure drop in the reactor, due to increased feed composition, which offsets increased propylene partial pressure, thereby preventing lower acrylic acid selectivity resulting from higher propylene pressure; and
(6) interstage problems are substantially eliminated.
In one aspect of the present invention, there is provided a process for the vapor phase oxidation of propylene to acrylic acid, comprising the steps of: (A) feeding a reactant composition comprising: (i) greater than 7 percent by volume propylene, (ii) oxygen, (iii) water vapor, and (iv) the remainder including a major amount of at least one inert gas, into a reactor; the reactor including a plurality of contact tubes, containing at least one catalyst, disposed in a shell, wherein the inside of the reactor shell is divided into at least first and second heat transfer zones through each of which a heat transfer medium passes and each contact tube comprises two or more reaction zones capable of effecting the preparation of acrylic acid from propylene, and (B) contacting the reactant composition with the two or more reaction zones to form a mixed product gas comprising acrylic acid.
In a second aspect of the present invention, there is provided a process for the vapor phase oxidation of propylene to acrylic acid, comprising the steps of: (A) feeding a reactant composition comprising: (i) propylene (ii) oxygen, (iii), water vapor and (iv) the remainder being a major amount of at least one inert gas and a minor amount of at least one inert gas suitable for use as a fuel, into a reactor; the reactor including a plurality of contact tubes, containing at least one catalyst, disposed in a shell, wherein the inside of the reactor shell is divided into first and second heat transfer zones through each of which a heat transfer medium passes, wherein each contact tube comprises two or more reaction zones capable of effecting the preparation of acrylic acid from propylene, and (B) contacting the reactant composition with the two or more reaction zones to form a mixed product gas comprising acrylic acid.
In a third aspect of the present invention, there is provided a process for the vapor phase oxidation of propylene to acrylic acid, comprising the steps of: (A) feeding a reactant composition comprising: (i) greater than 7 percent by volume propylene, (ii) oxygen, (iii) water vapor, and (iv) the remainder being a major amount of at least one inert gas, into a reactor; the reactor including a plurality of contact tubes, containing at least one catalyst, disposed in a shell, wherein the inside of the reactor shell is divided into first and second heat transfer zones through each of which a heat transfer medium passes cocurrent to the reactant composition flow, wherein each contact tube comprises reaction zones A and Axe2x80x2 which contain one or more catalysts capable of catalyzing oxidation of propylene to acrolein, reaction zones B and Bxe2x80x2 which contain one or more catalysts capable of catalyzing oxidation of acrolein to acrylic acid and a reaction zone C, containing a high surface area material having heat transfer properties and no percent catalyst, disposed between reaction zones Axe2x80x2 and B, wherein the reaction zones A and Axe2x80x2 have a different catalytic activity for converting propylene to acrolein and reaction zones B and Bxe2x80x2 have a different catalytic activity for converting acrolein to acrylic acid, and (B) contacting the reactant composition with the two or more reaction zones to form a mixed product gas comprising acrylic acid.
In a fourth aspect of the present invention, there is provided a reactant feed composition for vapor phase oxidation of propylene to acrylic acid in a single reactor, including: (i) 7.01 to 11 percent by volume propylene, (ii) oxygen in an amount suitable to provide an oxygen to propylene ratio of 1.6 to 2.2:1.0, (iii) 2 to 12 percent by volume water vapor, and (iv) the remainder comprising a major amount of at least one inert gas and a minor amount of at least one inert gas fuel.